CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > CFX

Negative value for static enthalpy calculation in CFD Post

Register Blogs Members List Search Today's Posts Mark Forums Read

Like Tree2Likes
  • 1 Post By ghorrocks
  • 1 Post By Opaque

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   April 29, 2014, 03:01
Default Negative value for static enthalpy calculation in CFD Post
  #1
New Member
 
Bostjan
Join Date: Apr 2009
Posts: 24
Rep Power: 16
brajh11 is on a distinguished road
Anybody know what exactly means negative value for static enthalpy in CFD Post?

I have seen in material definition and the reference state is 25 C and 1 atm.

For example:
areaAve(Static Enthalpy)@Outlet = -1.53639e+006 [J kg^-1]
areaAve(Static Enthalpy)@inlet = -2.5098e+006 [J kg^-1]
areaAve(Static Enthalpy)@Burner_1_Air_inlet = -2198.38 [J kg^-1]
brajh11 is offline   Reply With Quote

Old   April 29, 2014, 04:33
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,665
Rep Power: 143
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
Negative enthalpy simply means the enthalpy is less than the reference value. It is the differences in enthalpy which are important.
njdyck likes this.
ghorrocks is offline   Reply With Quote

Old   April 29, 2014, 05:03
Default
  #3
New Member
 
Bostjan
Join Date: Apr 2009
Posts: 24
Rep Power: 16
brajh11 is on a distinguished road
Where is possible to check the enthalpy for standard material (for example CO, CO2, H20, N2, O2,...) at reference state if material properties is selected in NASA Format?

Last edited by brajh11; April 29, 2014 at 08:39.
brajh11 is offline   Reply With Quote

Old   April 29, 2014, 08:50
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,665
Rep Power: 143
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
You define the reference point in the materials tab.
ghorrocks is offline   Reply With Quote

Old   April 29, 2014, 08:53
Default
  #5
New Member
 
Bostjan
Join Date: Apr 2009
Posts: 24
Rep Power: 16
brajh11 is on a distinguished road
I know. I have defined reference point at 25 C and 1 atm but I need to know the value for enthalpy at defined reference point - which value for standard materials (CO, CO2, H2O,....)if material properties is selected in NASA Format. How is possible to get this value?
brajh11 is offline   Reply With Quote

Old   April 29, 2014, 18:22
Default
  #6
Senior Member
 
Erik
Join Date: Feb 2011
Location: Earth (Land portion)
Posts: 1,166
Rep Power: 23
evcelica is on a distinguished road
Like Glenn said, the value is arbitrary, it is based off some arbitrary zero point. If you want to match something, NIST RefProp is a great source for fluid properties. I.m sure there are tables in textbooks and online as well.
evcelica is offline   Reply With Quote

Old   April 30, 2014, 01:25
Default
  #7
Senior Member
 
Join Date: Jun 2009
Posts: 1,788
Rep Power: 31
Opaque will become famous soon enough
Check your material definition, and reference state settings.

Under Material: MyMaterial, there should be a section named Reference State, and you can set, or check the following:

Reference Temperature
Reference Pressure
Reference Specific Enthalpy
Reference Specific Entropy

If your material uses the NASA format to define the specific heat capacity, the reference state is implicit within the coefficients, and you will have to evaluate the NASA expression at the reference state (usually 25 [C] and 1 [atm]..
Opaque is offline   Reply With Quote

Old   April 30, 2014, 02:41
Default
  #8
New Member
 
Bostjan
Join Date: Apr 2009
Posts: 24
Rep Power: 16
brajh11 is on a distinguished road
If under Material select reference specific enthalpy: -1.15334e+07 J/kg and reference specific entropy: 0 J/kgK at reference state 25 C and 1 atm, what is the value for specific enthalpy and entropy at 100 C? same value?
brajh11 is offline   Reply With Quote

Old   April 30, 2014, 03:53
Default
  #9
Senior Member
 
Join Date: Jun 2009
Posts: 1,788
Rep Power: 31
Opaque will become famous soon enough
A quick look at the ANSYS CFX Solver Theory Guide / Basic Capability / Documentation Conventions / Variable Definitions / Static Enthalpy will help understand how all the pieces are connected.

It explains how static enthalpy is computed for the different materials, and how the reference value is used.
njdyck likes this.
Opaque is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Plotting the Viscosity Contour in CFD Post ashtonJ FLUENT 3 June 2, 2015 13:06
Collect transient results CFD POST mortain CFX 15 April 17, 2015 09:29
How can I include scalable text in my images from CFD post? ziggo ANSYS 0 October 31, 2013 12:36
FEA results in CFD Post delaneyluke CFX 1 October 25, 2013 07:16
CEL mathematical functions in CFD Post Jonathan CFX 9 November 5, 2012 09:37


All times are GMT -4. The time now is 20:28.